Spontaneous Combustion

Photos and text by Rob Bicevskis

As with many of us, I have heard about spontaneous
combustion, but have never "experienced" it first hand.
There are stories about piles of damp hay igniting and
oil-soaked rags bursting into flame. I am always
fascinated by the heat generated by my compost pile. I
have measured the internal temperature to be as high as 160 F,
or 71 C.

After doing a bit of reading, it seems that oils
from grains and plants are more prone to spontaneous combustion
than petroleum oils. Linseed oil seems to be one of the
worst/best candidates, depending on whether or not you want a
fire.

The chemistry of spontaneous combustion is
that of oxidation. An oil in its liquid form will oxidize,
but there will be little heat buildup due to the limited surface
area. If we spread out the oil, then there is a large
exposure to oxygen, and more heat is generated. If we soak
oil into a rag of some sort and confine it to some degree, we
can end up with a magic (or tragic) combination which will
ignite. There are various factors that do have to be
properly balanced. Air needs to be able to get to the oil
to enable oxidation. If there is too much air, then heat
will be drawn away. If there is too little air, then there
won't be enough oxidation, and the ignition temperature may not
be reached. Having the whole rag/oil mass somewhat
insulated also helps to retain/build heat. Lastly, there
is a "critical mass" which must be established. A small
piece of cloth will lose too much heat to its surroundings.
One needs a large enough "pile" to make things work.

So much for theory, now let's try it out....

This was my
first try.

A coffee tin with a few pieces of old T-shirt.
Enough linseed oil was added to the rags to moisten
them, but not so much that any oil would drip off.

A digital multimeter was used with a thermal
probe to monitor temperature.

After three
hours, the temperature of the rags had gone up to
178 C or 354 F.

Along the way, various fumes could be seem coming
from the can. Sometimes it seemed like there
was a bit of smoke.

From this point on, the temperature of the can
decreased.

No fire. (Which was good since the coffee
tin wasn't in the best location for too much heat!)

This is what
the rags looked like after the experiment.

My thoughts after this first experiment:
More rags for critical mass. More air for more
oxidation. A safer location.

Experiment
#2:

More Air.

More rags for
critical mass.

A safer location.

Digital download of temperature data.

After about 4
hours, the temperature of the rags had reached 212 C
or 415 F. The picture shows some fumes/smoke
coming from the rags.

More smoke.

Temperature was 311 C or 593 F !

Note also the browning of the rags at the edge of
the can.

Now we're
cookin.

Total time since start: 5 hours.

Temperature 370 C or 700 F !

After about 7
hours, the temperature was leveling off at 431 C or
808 F !

I decided to lift a layer of the cotton to see
what was going on underneath. It looks like
the cotton had been slowly charring/burning for a
while.

With the
inner layers exposed, a bit of wind blew the
smoldering mass into flame.

Which quickly grew in size.

Performing this experiment in a safer location
was clearly a good move!

Now for some science:

The graph above shows the temperature -
time profile for the successful experiment. It
took about 3 1/2 hours for the whole mass to start
to heat significantly. Once it got going, the
temperature rise was pretty dramatic. I don't
know where the cotton actually started to burn.
I was guessing that it would be clear from the graph
- but it doesn't appear to be. Maybe this was
due to the temperature probe being buried in layers
of cloth that were not burning - and hence they
acted as an insulator.

It is interesting to compare the above numbers to
the graph above. The only real conclusion from
my experiment was that the cotton did ignite at some
temperature below 431 C. From experiment #1,
178 C is not enough.

Conclusions

Yes, spontaneous combustion is real!

I now have a lot more respect for proper disposal
of oily rags. (Soak them in water, and/or
store them in an air-tight container.)

Yes, spontaneous combustion can be used to start
a fire - but it does take time!

Further research:

When did the cotton actually start to glow?
What other types of oils can be used?
My gut feel tells me that even more air was needed.
A can is not a good insulator. Something like
cardboard would be better for heat retention.
How much cotton is needed for "critical mass?"
How big a role does the outside air temperature
play?